Can't find the "Reassign loop parameter" node in animation nodes addon
This is not a node you can directly invoke with the SHIFT + A submenu. Here is how to do it :


Create your loop and click it to select it
Assign a parameter
Go to the right-side panel (shortcut N, or click on the arrow on the right side of the animation nodes node tree)
Go to the "Node" subpanel
Click on the "Reassign" button next to the parameter socket
Click in the node tree window where you want to place the "reassign" node




If you want to have the "Condition" boolean operator in the node, you should :


Select the "Reassign" node
Go to the "Item" subpanel on the right
Turn on the computer screen icon next to the "Condition" input socket.


This works for all the animation nodes which may have hidden input/output sockets